Thank you very much @cabbit_girl; @Twinstar; @darkfaith; @lateChoco; @sumire007; for trying to help me solve the black layer issue.
Thanks God and my very persistent technician, my PC's finally fixed.
The problem was caused by the warsan ( diebold ) software that invaded my computer following a banking software installation.
When it was removed, the forum started working properly immediately. I've read online that the problem can happen again and again, as the software re-installs itself after a few days. It can also cause a lot of other problems to your PC.
For now, everything's OK but, of course, I'm gonna make a formal complaint to the bank.
I don't know if that's happening in other countries other than Brazil.
Well, I guess that's all.
Please, everybody, feel free to ask me more about the issue, if necessary.
And thank you very much again! ^^v